There are things parents can do to discourage children from having sexual
experiences at 11, writes Cassandra Jardine.

Getting changed after swimming this week I was listening to some girls chatting. They were only eight or nine years old but they were talking loudly and confidently about penises. Had they been tittering about "willies", I wouldn’t have been surprised, but their lack of embarrassment took me aback.

It is surely a good thing if children are now so well educated in human biology that they can talk knowledgeably about body parts and functions from an early age. Ignorance is no protection from anything, let alone teenage pregnancy or sexually transmitted diseases. The worry is that knowledge of the theory of sex is leading to a desire to experiment at an ever earlier age. Certainly that could be inferred from a new poll conducted by Sugar magazine in which 6 per cent of girls claimed to have had sex before they turned 12.

Help. I don’t consider my ten year-old, nor indeed my 13- or 14-year old, mature enough to be having intercourse. They seem far too young to handle the very different kind of relationship that ensues when flirting is replaced by physical intimacy.

Nor do I want them to feel too pressured by concerns about how they look that they become anorexic. The latest figures certainly suggest that this is striking increasingly young children: between 1996/7 and 2006/7 the number of under-16s admitted to hospital with anorexia jumped from 256 to 462. A greater understanding of eating disorders could explain the rise but I suspect, from hearing young girls talk endlessly about diets and appearance, that they are painfully self-conscious.

The problem is, in part, outside adult control. During the 20th century the age of puberty dropped by three years - or twelve days for each year - so many girls (and a few boys) are now maturing physically while at primary school. It follows that they will have an urge to experiment. But that doesn’t mean that there is nothing a parent can do to protect children from early sexual experience.

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For one thing we can help to delay puberty by not allowing children to get overweight. For girls the threshold is roughly 47 kilos, with boys it is 55 kilos. Early onset of puberty is also linked to stressful family circumstances, so if we stay calm and keep them from stuffing on sweets and pizzas, they may remain deaf to the siren song of sex for a little longer.

But avoidance cannot be the whole answer. I’m grateful to a zoologist, David Bainbridge who has written a scientific book about adolescence - Teenager: a natural history - for providing some useful facts that might just make a child think twice. At the earliest opportunity, I shall be dropping the two following bombshells into conversation. The first is that teenagers with lower IQs tend to start having sex younger. The second that 78 per cent African American girls surveyed felt that they were too young when they first had sex.

While I wait for those snippets to sink in, I shall console myself with another encouraging fact from the book, viz that times of economic prosperity coincide with periods of sexual liberty. Now we are in recession, one happy result could be that the age at which children become sexualised also recedes.